Ferroelastic crystals switched by motion of a domain wall having a zigzag configuration

ABSTRACT

Described are devices employing a ferroelastic crystal having two spontaneous strain states separated by a zigzag domain wall which can be switched to increase one strain state at the expense of the other by motion of the zigzag wall as a whole. Zigzag walls can be created by applying high stress to ferroelastic crystals under conditions which inhibit formation of normal planar walls. Once formed the zigzag walls are stable in the absence of applied stress and can be moved through the crystal by conventional means. The use of such a device in an optical shutter is also described.

United St:-

Flippen FERROELASTIC CRYSTALS SWITCHED BY MOTION OF A DOMAIN WALL HAVINGA ZIGZAG CONFIGURATION Richard B. Fllppen, Wilmington, Del.

E. I. du Pont de Nemours and Company, Wilmington, Del.

Dec. 26, 1972 inventor:

Assignee:

Filed:

Appl. No.: 318,502

US. Cl... 350/150, 350/151, 350/160,

Int. Cl. G02! 1/26 Field of Search 350/150, 151, 160, 287, 350/161;23/301 SP, 305, 293; 423/21, 263;

References Cited UNITED STATES PATENTS 8/l97 2 Kumada 350/150 Mar. 26,1974 5/1973 Barkely 350/150 8/1971 Kaspareck 350/287 Primary Examiner--Ronald L. Wibert Assistant Examiner-Michael J. Tokar Described aredevices employing a ferroelastic crystal ABSTRACT having' twospontaneous strain states separated by a zigzag domain wall which can beswitched to increase one strain state at the expense of the other bymotion of the zigzag wall as a whole. Zigzag walls can be'created byapplying high stress to ferroelastic crystals under conditions whichinhibit formation of normal planar walls. Once formed the zigzag wallsare stable in the absence of applied stress and can be moved through thecrystal by conventional means. The use of such a device in an opticalshutter is also described.

10 Claims, 10 Drawing Figures STRESS PA ENTEBM 3.799.648

I sum 3 or 3 i FIG. 5a FIG. 5!:

I/SWITCHING TIME BACKGROUND OF THE INVENTION 1. Field of the InventionThis invention relates to devices utilizing the switching properties offerroelastic crystals wherein the crystal is switched from one state ofspontaneous strain to a second and different state of spontaneousstrain.

2. The Prior Art I The existence of ferroelasticity was first clearlyrecognized by Aizu, J. Phys. Soc. Japan, Vol. 27, page 387 (1969).According to Aizu, a crystal is said to be ferroelastic when it has twoor more spontaneous strain orientation states in the absence ofmechanical stress and can be switched from one orientation state toanother by mechanical stress. The states are identical or enantiomorphicin crystal structure and different in mechanical strain tensor at nullmechanical stress. A plot of stress versus strain for such materialsexhibits a hysteresis loop similar to that of ferromagnetic materials.Also, like ferromagnetic materials, ferroelastic materials usuallyexhibit a Curie temperature above which the ferroelastic properties areabsent and a new phase of different crystal structure is present.

Ferroelastic materials are divided into domains throughout which thestrain tensor is the same. Two domains which differ in the strain tensorgenerally interface at one of two possible mutually perpendicular domainwalls which tend to be highly planar, and lie in distinctcrystallographic planes. Such walls tend to extend completely acrossthe'crystal. In switching, one domain grows at the expense of aneighboring domain. With walls of the type hereinabove discussed,switching is usually accomplished by motion of the domain walls in adirection perpendicular to their plane.

The different strain states are thennodynamically.

equivalent and equally stable. The phenomenon of switching takes placebecause the energy barrier between the states is small. This impliesthat the ferroelastic strain states are each a slightly distorted formof a certain prototype state of higher symmetry. In most cases, theprototype state is the state of the crystal above the Curie pointtransition temperature. The symmetry of the prototype state can bededuced from the symmetry of the ferroelastic state and the domainstructure (Aizu, J. Phys. Soc. Japan, Vol. 27, 387 (1969)) and hence thepossible species possessing ferroelasticity can be classified in termsof the point group symmetry of the prototype and the point groupsymmetry of the ferroelastic species. In Aizus notation, the point groupsymmetry of the prototype species is first written followed by F andthen the symmetry of the ferroelastic species.

from one orientation or state to another by the external application ofan electric field. Ferroelectric materials exhibit 'a hysteresis loop ina plot of electric polarization versus electric field and generallydisplay a transi tion temperature called the Curie temperature abovewhich the material is paraelectric.

Aizu (op cit.) has recognized coupled ferroelastic/- ferroelectricmaterials wherein ferroelastic domains coextensive with theferroelectric domains exist and which have the same Curie temperaturefor the ferroelectric and ferroelastic properties. Such crystals can Ibe switched by the application of either electrical or mechanicalstress. Domain structures and domain walls are, however, dictated by theferroelastic properties.

Crystals having coupled ferroelectric/ferroelastic properties have beenemployed heretofore in devices utilizing the switching propertieswhereby the crystal can be switched from one ferroelectric/ferroelasticstate to another, by an electric or mechanical stress and wherein thechange in state is detected by electrical or optical means as shown, forexample, in U. S. Pat. Nos. 3,623,031, 3,661,442, 3,559,185, 3,602,904,and

The switching of crystals having ferroelastic properties by thenucleation and lateral motion of planar do- I main walls is a relativelyslow process. Thus, utilizing the optical properties of the domains, ithas been proposed to construct optical shutters of the ferroelastic/-ferroelectric material gadolinium molybdate. The speed of such shutters,however, is limited to about the speed attainable with conventionalmechanical ,shut

ters.

SUMMARY OF THE INVENTION The present invention comprises a crystal of amatev rial having ferroelasticproperties, said crystal having a domainwall extending across the crystal in a zigzag configuration, and meansto move said domain wall whereby a substantial portion of said crystalis switched from one ferroelastic strain state to a second ferroelasticstrain state.

Preferred crystals having ferroelastic properties are coupledferroelastic/ferroelectric crystals and especially those crystalsexhibiting uniaxial spontaneous electric polarization. Particularlypreferred are crystals of the rare earth molybdates having theB-gadolinium molybdate structure, especially B'-gadolinium molybdateitself. This invention also comprises methods of making the aforesaidzigzag domain walls in crystals having ferroelastic properties byapplying a shear stress to such crystals under conditions inhibiting theformation of ordinary planar domain walls. Such methods include:

l. Clamping regions adjacent opposite edges of a single domain crystalof a material having ferroelastic properties to inhibit deformation ofthe crystal in the clamped regions, the edges of the clamped regionshaving boundaries with the central unclamped region parallel to atwinning plane in the materialand applying an increasing shear stressacross the unclamped region of the crystal parallel to the saidboundaries until domain walls form having a zigzag configuration.

2. Applying an increasing bending stress across the single crystal of amaterial having ferroelastic properties, said crystal being'divided intoa first domain and 'a second domain by a normal planar domain wall, the

bending shear stress being perpendicular to said do- THE DRAWINGS ANDDETAILED DESCRIPTION OF THE INVENTION There has now been discovered anew type of domain wall which can be induced in crystals havingferroelastic properties, and which is stable in the sense that, oneformed, such walls exist in the absence of applied electrical-mechanicalstress and can be moved back and forth in crystals as an entity bystress orin the case of coupled ferroelectric/ferroelastic species, byelectrical fields. Because of their appearance and properties, this typeof wall has been named a zigzag wall. The mobility of zigzag walls issubstantially greater than that of normal walls; indeed wall mobilitesthirty times or more greater than conventional walls have been achieved.

This invention will be better understood by a reference to the drawingswhich accompany the specification and in which:

FIG. 1 is a microphotograph ofa zigzag wall in a halfwave plate ofgadolinium molybdate.

' FIG. 2 is a microphotograph of a zigzag wall in a quarterwave plate ofgadolinium molybdate.

FIG. 3 is a sketch of the zigzag wall with the principal dimensionsindicated. 7

FIG. 4 illustrates one method of creating zigzag walls in a plate of acrystalline material having ferroelastic properties.

FIG. 5a, 5b and 5c illustrate another method of making a zigzag wall.

FIG. 6 is a graph illustrating wall velocity versus applied stress for adomain wall in a ferroelastic materials FIG. 7 illustrates the motion ofthe zigzag wall across the crystal.

FIG. 8 shows an optical switch using a crystal of a material havingferroelastic properties and containing a zigzag wall of the presentinvention.

In FIG. 1 is shown a microphotograph ofa zigzag wall in a half-waveplatemade of B-gadolinium molybdate which is a coupledferroelectric/ferroelastic material having Aizu species 42mFmm2. Theplate is cut perpendicular to the c axis, that is, the [001 1 axis ofelectric polarization. The photograph was made with polarized light withboth polarizer and analyzer aligned parallel to one of the [110] planeswhereby light transmitted by both domains is extinguished. Thepolarization properties of the crystal plate in the vicinity of thedomain wall differ from those of the surrounding domains when the domainwall is observed as a white line on a dark field. The general line ofthe zigzag wall is parallel to a l 101 direction and the wall movessubstantially as an entity along the other of the [I10] planes, onapplication of either electrical or mechanical stress tending to favorone of the domains separated by the wall.

FIG. 2 is a microphotograph of a quarter-wave c-cut plate ofB-gadolinium molybdate divided into two domains by a zigzag wall. Thephotograph is taken with the [1 10] set of planes. The crystallographicdirections I are indicated on the figure. The plate I is divided intothree domains, 2, 3 and 4, by a planar domain wall 6 and a zigzag wall5, the edges a, c and e. etc. forming the tips of the zigzag aresubstantially aligned in a plane I which is a twinning plane capable ofsupporting a normal domain wall such as a domain wall 6. Likewise. the

edges b, d andfalso lie in a plane parallel to the twinning plane of thenormal domain wall, 6. The distance between the planes containing theedges 0. c. e and the plane containing the edges b, d, f will be calledthe width of the wall, w. Width w can vary widely, even for the samematerial. For example, in gadolinium molybdate, w can be from to 4,000microns. The distance between the edges of the zigzag such as a to c, cto e, b to d, or d tofare highly uniform for a given wall. This distancecan be called the pitch, p. Again, the pitch can vary substantially evenin crystals of the same material. For gadolinium molybdate, the pitch isbetween 5 and- 150 microns.

The ratio of the pitch to the width is less variable and is generallybetween 0.05 and 0.15. The walls, such as the wall between edge a andedge b which form the zigzag, appear to slightly sygmoid in shape butcan be approximated closely by planes. The planes lie at equal anglesfrom the (I I0) twinning plane perpendicular to the twinning plane inwhich domain wall 6 lies. Assuming that the zigzag wall is composed ofplanar segments, the angle 0 can be calculated from the relation to tan0 p/2w, thus from the aforesaid values of Turning now to FIG. 4, in FIG.4 is shown a c-cut crystal of gadolinium molybdate 10 having its edgescut parallel to the [110] planes. A fixed clamp 11 is cemented along oneedge of the crystal and a movable clamp 12 is cemented along theopposite edge so that the edges of the clamps are parallel to (l 10)planes. A cement should be employed which can be applied in the liquidstate and hardened without substantial shrinkage so that strains are notimposed upon the crystal by hardening of the cement. The crystal isplaced upon the clamps and cement is allowed to flow along the edges ofthe crystal by capilliary action in order to form a linear cement linebetween the edge of the clamp and the crystal. The regions of thecrystal cemented to the clamps l1 and 12 are prevented from deformingandthus switching from one domain state to another; Thus domain wallstrapped within the free region of the crystal are retained therein bythe clamps 11 and 12. Clamp II is provided with a screw 13 bear- 7 ing aclamp 12 so that pressure can be applied at clamp 12 as desired. Also,if desired, a'strain gauge can be placed between the screw 13 and clamp12 to measure the applied stress, although this is not essential. In onemethod of using the apparatus of FIG. 4, crystal 10 is a single domaincrystal. If the crystal has ferroelectric properties, it must beelectroded on the faces intersecting the ferroelectric axis andconnection made between the electrodes so that the material may bemanipulated like a pure ferroelastic material. Stress is then applied tothe crystal 10 by tightening screw 13 against clamp 12. When a certainlevel of stress is attained, two zigzag domain walls 14 and 15 will formin the crystal adjacent sudden appearance of the domain walls isaccompanied by an audible noise. If only one of the domain walls isrequired, clamp 12 may be removed by dissolving the cement in a suitablesolvent. Domain wall 15 can then be expelled from the crystal by theapplication of stress.

Another method of making the zigzag walls of the present invention isillustrated in FIG. 5a, 5b and 50. In FIG. 5a, the crystal consists of ac-cut plate of gadolinium molybdate having its edges parallel to the 110planes and is divided into two domains by a domain wall 21. The crystalis placed in a frame 22 between rounded protruding lugs 23 and 24 and ascrew 25 passing through the frame 22 so that a bending stress can beapplied'to the crystal perpendicular to domain wall 21, on applicationof pressure with screw 25. In FIG. 5a, the crystal is shown prior toapplication of bending stress. In FIG. 5b, the same crystal is shownafter bending stress is applied. Blade-like domains 26, 27, 28, 29

and 30 are formed in the crystal perpendicular to the domain wall 21 andimpinge thereon. The domain wall 21 appears to bend slightly asindicated where the bladelike domains impinge on it. When a criticalstress is exceeded, the domain wall 21 suddenly transforms to a zigzagdomain wall as shown in FIG. 5c and the bladelike domains disappear.

The stress required to form the zigzag domain wall is substantiallygreater than the stress required to nucleate a planar wallin thegadolinium molybdate crystal. Typically, the zigzag domain walls areformed at pressures of 5 kg per sq. cm or more in gadolinium molybdate.Application of further stress tends to decrease the width and to alesser extent the pitch of the zigzag wall. The exact levels of stressrequired varies from crystal to crystal even in the same substance.However, for a given crystal, the results appear quite reproducible.

Once formed, the zigzag wall can be manipulated by an electrical ormechanical stress in the same manner as a normal plannar domain wall.Methods of manipulating domain walls in crystals offerroelectric/ferroelastic materials are described more completely inthe copending commonly assigned application ofJohn R.

Barkley, Ser. No. 251,055. The variation of the speed of switching thecrystal by movement of the zigzag wall as a function of applied stressis shown in FIG. 6. Over the range of velocities measureable bystroboscopic techniques, i.e., between points I and m on the curve ofFIG. 6, the rate of switching measured by the reciprocal of theswitching time is a linear function of stress.

A certain threshold stress isrequired before apprezigzag domain wallscompared with the normal planar walls is not fully understood, it ispossible that it is due to a geometric factor.

FIG. 7 illustrates the geometric factors associated with the movement ofthe domain wall. In FIG. 7, a portion of the zigzag wall 40 is shown atan initial location and the same wall 41 is shown after motion forwardin response to stress. The motion of the essentially planar domain wallsmaking up the zigzag wall normal to the length is indicated by a vectorshown by arrow 42 whereas the motion of the zigzag wall normal to thelength of the zigzag wall is given by a vector indicated by the arrow43. The ratio of vector 43 to vector 42 is given by cosecant 0 wherein 6is as defined herein:

above. The velocity of the essentially planar walls making up the zigzagwall normal to their face calculated from this expression is found to beessentially that of a conventional planar wall extending across thecrystal.

For example, in a mixed molybdate of gadolinium dysprosium, the zigzagwall is formed in a crystal wafer 0.75 mm thick and 5.5 mm wide with thespace 5.0 mm

long between clamps. The zigzag wall had a width w of 0.47 mm and apitch of microns between the tips of the zigzag. The wall moved at athreshold voltage of 50 volts applied to electrodes on the faces of thewafer, that is, a threshold field of 650 volts per centimeter. Measuredmobility of the wall was found to be 0.280 cm sec-volts-, which is about25 times the mobility measured for a planar wall in a crystal of thismaterial. On the above considerations, an increase im mobility of about15 would be expected More than one zigzag wall can be formed in acrystal and such walls can exist together with normal planar walls inthe crystal provided the walls do not intersect. If the material hasferroelectric properties a crystal can be divided into the zones bypartial electroding and the walls moved independently within each zoneby application of suitable voltages to the partial electrodes.

FIG. 8 illustrates an arrangement which cam be employed as an opticalswitch. In FIG. 8 there is shown a source of light 50, a lens SI adaptedin a range to collimate the light, a polarizer 52 in the path of thecollimated light, and an aperture stop 53 to'limit the aperture of thedevice. A plate of a ferroelectric/ferroelastic material such asgadolinium molybdate 54 having a zigzag wall therein 55 is equipped withelectrodes 56 and 57 on the c-cut faces thereof perpendicular to theplanar zigzag wall. Electrode 56 is a transparent electrode coveringsubstantially all of the face of the crystal, whereas electrode 57 is arectangular. electrode covering the face of the crystalalong the lengthof the zigzag wall, but having edges parallel to the zigzag wall whichare displaced from the edges of the crystal 54. Thus on application of avoltage between electrodes 56 and 57 an electric field is created in thecenter of the crystal tending to move the zigzag wall but no field iscreated on the edges of plate 54 so that the zigzag is confinedsubstantially to the region of the plate covered by electrode 57. Avoltage source 58 is supplied to provide an electric field of variableintensity and polarity between electrodes 56 and 57 whereby the domainwall 55 can be driven across the crystal 54 in either direction asdesired. Crystal 54 is desirably cut to a thickness which provides forquarter-wave retardation for light traversing the domains of thebirefringent ferroelectric/ferroelastic crystal. The light passingthrough crystal 54 passes through a second quarter-wave plate 59 andthen through an analyzer 60.

Plate 59 can be a quarter-wave plate of gadolinium molybdate in the formof a single domain or a quarterwave plate of quartz or otherbirefringent material.

The ferroelectric/ferroelastic domains of gadolinium molybdate arebiaxially birefringent with A n 3.90 X 10' for A 0.5 .1.. A quarter-waveplate is approximately 0.37 mm in thickness. Plane polarized lightpassing through the crystal 54 is converted to circularly polarizedlight, the sense of the circular polarization being reversed onswitching the crystal from one ferroelastic strain state to the otherferroelastic strain state I by motion of domain wall 55. The circularlypolarized light emerging from plate 54 is converted to plane polarizedlight by passage through plate 59 wherein the plane of polarization liesin one of two directions at right angles to each other according to thestate to which crystal 54 is switched. Analyzer 60 is then set toextinguish light passed byone or other of the two states of crystal 54separated by domain wall 55. Accordingly, by application of anappropriate voltage from source 58, light from source 50 can be eitherpassed or blocked by the optical system.

Because the zigzag walls of the present invention have a substantialwidth, they are employed in optical shutters wherein the opticalaperture is large compared with the width of the wall. The increase inspeed of operation compared with an ordinary planar domain wall is givenby the expression (Sz/Sp) (m /mp) (w A/A) where Sz/Sp is the ratio ofswitching speeds for a given field. m mp is the ratio of mobilities forzigzag and planar walls. A is the width of the aperture and w is thewidth of the zigzag wall. As noted above, values of m /mp of about 30are readily obtained, thus even where the aperture is equal to the widthof the zigzag wall an increase in switching speed in optical switches,of about [5 can be obtained by the use of the switching element of thepresent invention.

The existence of properties of zigzag domain walls depends on theferroelastic properties of the material.

Accordingly, the existence of ferroelectric properties coupled with theferroelastic properties is not necessary in this invention. It coupledferroelectric/ferroelastic crystals are employed, it is preferred thatthey should exhibit uniaxial behaviour. That is, the electricpolarization must be constrained in one direction or the other along thespecific axis. In addition to this, the twinning planes'should have onlya finite number of specific orientations within the crystal so thatdomain walls can be fonned, capable of being moved in a controlledmanner by external control of the electric field, or with mechanicalstress. For the purposes of this invention, therefore, whenferroelastic/ferroelectric single crystals are employed, they shouldexhibit uniaxial electric polarization. Such crystals include all thecrystals in the following Aizu point groups: 42mFmm2, v

4F2, 222F2, 42mF2, 422F2, 622F2, 43mFmm2, and 23F2. Most preferred arecrystals having the gadolinium molybdate structure, Aizu species 42mFmm2which can be represented by the formula More specifically it is theferroelectrie/ferroelastic phase commonly referred to as [3' phase ofthe gadolinium molybate type materials which exhibits coupledvferroelectric/ferroelastic behaviour. These materials display twoorientations of twinning planes which are normal to both two-foldorientation axes of the paraelectric group 42m. The electricpolarization vector transition to the mmZ ferroelectric phase and theybecome the twinning operations that interconvert theferroelectric/ferroelastic domains-In particular, crystals having theformula B-X (MIO,) where X can be Md, Sm, Eu, Gd or'Tb and the mixedrare earth molybdate DyGd(MoO Mobile zigzag domain walls have also beenobserved in pure ferroclastic materials, i.e., materials in which. theferroelastic property is not coupled to another crystal property such asferroelasticity. Thus a-lead phosphate is a pure ferroelastic material,Aizu species 3mF2/m, which has a Curie temperature of 179 above whichthe crystal point group is 3m and below which the crystal point group is2/m. In the ferroelastic state a strain occurs in one of the threeequivalent mirror planes of the prototype, high temperature trigonalphase resulting in one of three possible orientations for thenionoclinic axis and thus three possible domains throughout which themonoclinic axis has the same orientation. Each pair of domains caninterface at one of two possible, mutually perpendicular domain wallswhich lie along the direction of the domain interfaced. There are thussix possible domain walls which ,divide into two types: three n-wallsoriented essentially perpendicular to the be plane (corresponding to thec plane of the trigonal form) and oriented at 60 to the ac plane of eachdomain: and three t-walls each tilted at an angle of about 73 to the beplane and oriented at 30 to the ac plane of each domain. The spontaneousstrain appears as a bend" a of about 4.4 across one n wall and a bend Bof 1.6 in the plane perpendicular to the n wall in the be plane. For a twall the bend of the crystals across the wall is 4.6 and the be planesof the domains are essentially colinear.

For a plate of a-lead phosphate in the ferroelastic state cut parallelto the be plane at a thickness of 0.3 mm a mobile zigzag t wall has beenobserved with p 18p.'and w 0.6 mm, i.e., having substantially the samedimensions as zigzag walls observed in various rare earth molybdatecrystals.

a-lead phosphate is transparent from 0.28 to 5p. The refractive index isabout 2. l the material being biaxially birefringent ()An 7 X 10' in thebe plane and is thus suitable for the construction of mechanicallyactuated optical switches.

Since obvious modifications and equivalents in the invention will beevident to those skilled in the arts, I propose to be bound solely bythe appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A device comprising a crystal of a material having tion and means tomove said domain wall whereby a substantial portion of said crystal isswitched from one I wherein R and R are scandium, yttrium or a rareearth element having an atomic number of from 57 to 71, x

is from to L0 and e is from 0 0.2, and having the' B'-gadoliniummolybdate stucture.

5. Device of claim 4 wehrein said crystal is cut wit faces perpendicularto the c-axis.

6. Device of claim 5 wherein said means to move the domain wallcomprises electrodes on the faces perpendicular to the c-axis and meansto apply an electric voltage to said electrodes.

7. An optical shutter comprising a transparent birefringent crystal of amaterial having ferroelastic properties, said crystal being divided intoa first ferroelastic domain and a second ferroelastic domain by a domainwall extending in a zigzag configuration across said crystal,

means to move said domain wall across a substantial portion of saidcrystal whereby said portion is switched to the other domain, andpolarizing means on one side of said crystal and analyzing means on theother side of said crystal arranged to extinguish light transmittedthrough the assembly of said polarizer, said crystal and said analyzerwhen said first domain is in the opticl path. 8. The optical shutter ofclaim 7 wherein the aperture is large compared with the width of thedomain wall.

9. The optical shutter of claim 8 wherein said crystal has theformulawherein R and R are scandium, yttrium or a rare earth element having anatomic number from 57 to 71, x is from 0 to 1.0, and e is from 0 to 0.2;and having the B'-gadolinium molybdate structure, said crystal being cutas a plate with faces perpendicular to the c-axis.

10. Apparatus of claim 7 wherein said plate is cuton a )./4 plate andsaid analyzer is a circular analyzer.

U.S. 3,732,5 i9.--

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,799,

i DATED March 26, 1 7

INVENTOR(S) Richard B. Flippen It is certified that error appears in theab0ve-identified patent and that said Letters; Patent are herebycorrected as shown below;

Cover page, References Cited "Barkely" should be --Barkley-.

Col. 3, line 8 "one" should be --once.

Col. 3, lines 21, 23, 39, and 56 each instance "microphotograph" shouldbe --photomicrograph-- Col. 4, line Col. '4, line insert --6-.

line

001. 4, line with --there-.

Col. 5, line Col. 5, line Col. '5, line Col. 6, line line Col. 6,

l0 "planes" should be --plane.

26 between "angles" and "from" 30 between "relation" and "tan" 3H delete"in Fig. 4" and replace 32 "levels" should be ----level---.

41 after "251,055" insert --,now

66 second "the" should be -their--'-.

9 between "gadolinium" and "dysprosium" 20 "im" should be in--.

21 "15" should not be in bold face type.

38 "cam" should be --can--.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO.3,799,648

DATED 1 March 26, 1974 |NVENTOR(S) Richard B. Flippen i it is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Col. 6, lines #6 and L7 "planar" should be -plane Of the''- I Page 2 rCol. 7, lines 28 and 29 "(Sz/Sp) (m /mp) (w A/A) should read "5e iie A001. 7, line 29 "Sz/Sp" should read s -r- Col. 7, lines 30 and 33 eachinstance "m Should be m mp Col. 7, line 36 between "15" and "can" insert--times--.

Col. 7, lines 55 and 56 change i-2mFmm2 41 2, 222F2, i2mF2, 42252,622F2, i mFmm2" to read r2mFmm2, i1 2, 122232,

121111 2, 4-221 2, 622F2, 3mFmm2--.

Col. 7-, l1ne. 58 change "42mFmm2" to read i2mFmm2--.

Col. 7, line 6 4 between "x" and "from" insert --is--.

Col. 8, line 2 after "phase" (each occurrence) insert 'Col. 8, line 7insert a bar so i-2m" should be "3 2m".

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. I

DATED March 26, 1974 Page 3 INVENTOR(5) 1 Richard B. Flippen It iscertified that error appears in the above-identified patent and thatsaid Letters Patent r are hereby corrected as shown below:

col. 8, line l t the 2 in "mm2" should not be in bold face type. a a

Col. 8, line l7 "s X (MlO should be Col. 8, line 18 "Md" should be--Nd--.

Col. 8, line 23 "ferroelasticity" should be -ferroelectricity.

Col. 8, lines 25 and 26 insert a bar over each 3 I 7 Claim t, Col. 9,line 15 formula should read I 0 "El-@32 3 l-e e 3 Claim t, Col. 9, line18 between "0" and "O .2"

insert -to-.

Claim 9, Col. 10, line 17 the formula should read o 3Mo W O Claim 10,,Col. 10, line 25 "7" should be --9--.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 a 799648 Dated March N 19 74 Richard'B. Flippen Inventor(s) Page 4 It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

FIGURES 3 and 5b should appear as shown below:

Signed and Scaled this twenty-third D ay Of September 1 9 75 [SEAL] Atrest:

RUTH C. MASON C. MARSHALL DANN l s k ff fl (rmzmixxw'nm'r uflarcnls andTrademarks

1. A device comprising a crystal of a material having ferroelastic properties, said crystal having a domain wall extending across the crystal in a zigzag configuration and means to move said domain wall whereby a substantial portion of said crystal is switched from one ferroelastic strain state to a second ferroelastic strain state.
 2. Device of claim 1 wherein said crystal is a crystal of a coupled ferroelectric/ferroelastic material exhibiting uniaxial electric polarization.
 3. Device of claim 2 wherein said means to move the domain wall comprises electrodes on the faces of said crystal intersecting the ferroelectric axis and means to apply a voltage to said electrodes.
 4. Device of claim 2 wherein said crystal is a crystal having the formula (RR''1 x)2O3 . 3Mo1 eWeO3, wherein R and R'' are scandium, yttrium or a rare earth element having an atomic number of from 57 to 71, x is from 0 to 1.0 and e is from 0 0.2, and having the Beta ''-gadolinium molybdate stucture.
 5. Device of claim 4 wehrein said crystal is cut with faces perpendicular to the c-axis.
 6. Device of claim 5 wherein said means to move the domain wall comprises electrodes on the faces perpendicular to the c-axis and means to apply an electric voltage to said electrodes.
 7. An optical shutter comprising a transparent birefringent crystal of a material having ferroelastic properties, said crystal being divided into a first ferroelastic domain and a second ferroelastic domain by a domain wall extending in a zigzag configuration across said crystal, means to move said domain wall across a substantial portion of said crystal whereby said portion is switched to the other domain, and polarizing means on one side of said crystal and analyzing means on the other side of said crystal arranged to extinGuish light transmitted through the assembly of said polarizer, said crystal and said analyzer when said first domain is in the opticl path.
 8. The optical shutter of claim 7 wherein the aperture is large compared with the width of the domain wall.
 9. The optical shutter of claim 8 wherein said crystal has the formula (RR''1 x)2O3 . 3Mo 1 eWeO3, wherein R and R'' are scandium, yttrium or a rare earth element having an atomic number from 57 to 71, x is from 0 to 1.0, and e is from 0 to 0.2; and having the Beta ''-gadolinium molybdate structure, said crystal being cut as a plate with faces perpendicular to the c-axis.
 10. Apparatus of claim 7 wherein said plate is cut on a lambda /4 plate and said analyzer is a circular analyzer. 